The long-term goal of the research of both the individual lab and the Program Project is to understand how the brain encodes sensory and motor representations critical for behavior, and how these are affected by experience and learning. The focus of the current proposal is the neural basis for auditory processing of complex sounds, which is critical for perception and communication in many species, including humans, but surprisingly poorly understood. This question will be tackled in the auditory forebrain of adult male songbirds, in the complex of areas that lies between the auditory thalamus and the specialized brain areas for song production known as the song system. These auditory forebrain areas will be neurophysiologically investigated using complex natural sounds, including primary birdsongs, because such sounds are important for birds both to learn their own songs and to discriminate between other individuals of the same species. The proposed experiments will test the hypothesis that the systematic use of this rich class of auditory stimuli, with strong ethological relevance to the animal, will reveal a hierarchy of complex and non-linear aspects of high level auditory processing that would not be evident without the use of these sounds, and that may include specialized representations of the bird's own and tutor songs. Experiments will use newly developed methods for quantifying neural responses to these sounds, including spectral-temporal receptive fields (STRFs) based on song responses, spike-triggered covariance matrices of responses to colony sounds (STCMs), and information theoretic analyses of stimulus and responses. In addition, neurons will be recorded with multiple electrode arrays, to investigate whether neural encoding of sounds in the auditory forebrain is accomplished with populations of neurons rather than with single, highly selective cells. In a second Aim, the same approaches will be used to test the idea that auditory representation is influence by acoustic experience during nearly life. Birds will be raised either in conditions of song deprivation or in an altered sound environment that lacks the normal spectral-temporal complexity of colony sounds, and the auditory forebrain of these birds will then be neurophysiologically studied and compared to that of normally-reared birds. Such of studies of normal and abnormal development in songbirds, in parallel with the related work of our colleagues, have the potential to reveal common principles underlying the neural effects of sensory experience and learning, with general relevance to sensory processing and its disorders in many animals, including humans.